By Svea Herbst-Bayliss NEW YORK (Reuters) – Southwest Gas Holdings reached a settlement on Friday with Carl Icahn that will replace the company’s chief executive and hand as many as four board seats to the billionaire investor. The settlement ends a months-long battle Icahn launched with the Las Vegas, Nevada-based company in October as it pushed ahead with plans to buy Questar Pipelines for roughly $2 billion. Southwest Gas said it promoted Karen Haller to CEO and president, replacing John Hester immediately. Icahn said in a statement that no settlement would have been reached without Hester …